1. Field of the Invention
This invention relates to piston and piston rod assemblies, particularly such assemblies as are used in relatively small sized hydraulic actuators, on the order of three inches in diameter.
2. Description of the Prior Art
U.S. Pat. No. 4,628,795 issued to J. Pickard, et al shows one environment in which the invention can be practiced. The patent shows a brake band for an automotive transmission, and a piston-cylinder assembly for driving the band to its engaged or disengaged positions.
At least one application uses a double motion piston and piston rod assembly to operate each transmission band. When pressurized liquid is supplied to the piston the piston slides along the associated piston rod against the action of a load spring carried on the rod. The piston impacts against the piston rod to drive the rod through its designated stroke for operating the brake band.
In the described system the piston is formed as a one-piece metal casting having a cup-shaped configuration. The casting includes a central sleeve section slidable on the piston rod, a radial end wall extending radially outwardly from one end of the central sleeve section, and an outer annular side wall extending axially from the radial end wall. The free end of the annular side wall is turned outwardly to define a peripheral lip.
A groove is machined into the edge of the peripheral lip to form a seating area for a seal member. During service, the seal engages the associated cylinder side wall to prevent liquid flow across the piston-cylinder interface. The casting process required to form the one piece piston is relatively costly. Additional cost is involved in machining the annular groove that seats the seal.
An alternative approach to manufacturing such a piston relied upon forming the piston as a one-piece stamping. Although one-piece stampings have previously been formed as cup-shaped pistons, the bend radii required to achieve desired die draw depths has precluded the use of one-piece stampings in many applications. That is, the large curved bend profiles needed to prevent tearing or cracking of the stamped sheet metal often interfered with other piston assembly components such as biasing springs and the like. For example, the curved surfaces of the bend formed between the end wall and side wall of the piston have prevented the proper seating and alignment of piston biasing springs which typically are seated on flat rather than curved surfaces.